Why Your Composting Toilet Doesn't Actually Compost Poop

Composting toilets are getting more popular – whether they are being used in RVs, cabins in the woods, or backcountry camping areas. But calling those toilets “composting toilets” is simply incorrect. These toilets don’t actually compost your shit. By the time your toilet is full and needs to be emptied, the poop in there hasn’t actually broken down to be actual compost. It’s still just shit mixed with toilet paper. This misconception could negatively affect a lot of people.

WHAT IS COMPOST?

It’s important to define what compost is first. According to the composting council (yes, they exist!), “Compost is the product resulting from the controlled biological decomposition of organic material that has been sanitized through the generation of heat and ‘processed to further reduce pathogens’, as defined by the U.S. EPA, and stabilized to the point that it is beneficial to plant growth. Compost bears little physical resemblance to the raw material from which it originated. Compost is an organic matter source that has the unique ability to improve the chemical, physical, and biological characteristics of soils or growing media.” 1

Composting Isn’t Easy

If you’ve ever had a compost bin in your backyard for kitchen scraps, you know that composting isn’t easy. I learned this after getting so excited about composting that Jerud bought a compost bin from his co-worker for me. I eagerly chose a spot in the backyard for the bin and started to collect our kitchen scraps. But I quickly learned that the easy part was collecting and tossing our scraps into the compost bin. Actually getting the stuff in the bin to start composting – not so much.

Jerud ended up taking over because, embarrassingly, I didn’t have enough patience to work at making the stuff actually compost. My fault aside, part of the issue was that using a one bin system doesn’t work very well. Food doesn’t turn into compost overnight and while it’s breaking down, we’re continuously tossing fresh scraps in. Eventually our kitchen scraps turned into good soil to add to our garden, but we’d still occasionally find pieces of eggshells and fruit skins in the mixture.

To make a composting toilet work correctly is just as much work as a regular yard compost bin.

How Does Something Turn Into Compost?

Since I never mastered actually being able to turn food scraps into compost, I can only tell you the theory behind composting (with help from Jerud and others).

Microbes are what makes poop/food scraps into compost. In a yard compost, they come from the soil your bin sits on top of, and in a composting toilet they get added from a starter packet. (We add microbial drain cleaner after each toilet empty.)

Think of microbes as little people: They need all the essential things we do to survive. Our organic waste provides the microbes’ food, but they also need water, oxygen, and warmth to survive. The microbes generate heat, and it’s that heat that kills pathogens in the compost bin. When conditions are right, it takes them months to turn food scraps into soil.

I’m simplifying the compost process to keep this post short. Please research “how to compost” or “how composting works” to get the full picture. This article by Planet Natural Research Center is a good place to start.

Why Your Composting Toilet Doesn’t Compost

When I say composting toilets, I’m talking about waterless toilets that have a container to hold poop. This container sits directly under the toilet seat (unless it’s being used on a large scale like national forest backcountry toilets) and is partially filled with sawdust/peat moss/coco pith.

We use coco pith for our composting toilet. This is what it looks like after water has been mixed in.

Below are the reasons composting toilets don’t usually compost poop.

Food

It’s crucial to strike the right balance between nitrogen and carbon for your composting to be successful. Nitrogen rich items are “green” stuff like vegetable and fruit scraps; in a composting toilet, nitrogen rich items are your turds. Carbon rich items are “brown” stuff like leaves; in a composting toilet, carbon rich items are sawdust, coco pith, or peat moss. Naturally occurring microbes need the right amount of nitrogen and carbon to feed off of. The desired ratio between carbon and nitrogen isn’t 50-50, much more carbon is needed (25 to 30 parts to 1).

It’s extremely important to have the right ratio of poop to sawdust/coco pith/peat moss in the toilet. This is hard to accomplish in yard composting and becomes a bit more difficult when you have a small shit-collecting container to start with, like a composting toilet. The ratio requires a lot more sawdust/coco pith/peat moss than poop. If your composting toilet is filled with the ideal ratio, it would mean you’d have to empty your shit container more frequently.

Liquid

The trap door is left closed when you pee into the toilet and pee gets divert down the two holes in the front. The trap door is opened when it's pooping time. (The toilet seat is lifted up in this photo.)

The composting process needs liquid for the microbes to survive, but too much will “drown” them and create an anaerobic environment. An anaerobic environment isn’t desired because it’s a slower composting process and it smells.

If the liquid is pee, the additional risk is that the ammonia will kill the microbes. Having too much liquid isn’t typically a problem in commercially available compost toilets because they’re made to divert pee from where the poop goes. This diversion and fans in the poop container minimizes smell by ensuring the turds aren’t too wet. But it could make it too dry which slows down the decomposition rate.

Warmth

To ensure the microbes stay active and alive in your toilet, it needs to be warm enough. Yes they create their own heat as they eat food scraps and poop, but if it’s too cold, they can’t keep up. The microbes may not die if it gets too cold, but they won't be as active which then slows down the composting process. The compost pile also needs to reach thermophilic conditions, which is more than 113 degrees, to start the pathogen-killing process. This can’t be guaranteed in an RV because they aren’t as well insulated and heated as a regular house. On top of that, the fan in the poop container ends up dispersing the heat that’s being generated by the microbes - overall decreasing the thermophilic conditions needed.

Retention

A composting shit container typically holds a month or less of poop (depend on variables like how big the bucket is, how many times a day it gets shat in, and how big those poops are). For reference, our Air Head composting toilet gets shat in 2 – 3 times a day/every day, and needs to be emptied every 3 weeks.

In a study done at Woods End Research Laboratory, samples taken from composting toilets that had a 1.5-year retention period passed the EPA’s pathogen test rules. It could potentially be quicker if ALL the conditions are perfect. The main pathogens that are of concern is E. coli and salmonella, and then there’s also roundworm – which is a more persistent pathogen. With adequate time and heat, poop will be properly composted to become a nutrient rich and safe material to use. But if a composting toilet can only hold a month’s worth of poop, there is no way that poop is actually composted to safe pathogen levels by the time the toilet needs to be emptied.

Based on the last bullet point alone, calling these toilets “composting” give users the wrong impression. Composting toilets can potentially start the composting process (again, if everything is done correctly), but the toilets don’t actually complete the process. Instead, the poop in the toilet would need to be placed in another container to give it enough time to turn into true compost. It’s because of this and the other reasons mentioned above that composting toilets should really be referred to as dry toilets - or even better, a reader called them "dessicating". (Note, composting toilet companies all add the caveat that the toilets do compost if used “correctly”. But “correctly” refers to the whole process.)

Clarification

We purchased a composting toilet mainly because it was a dry toilet and not so much because it has the ability to compost. Questions about marketing aside, these are still good dry toilets and meet our needs while living on the road. We know that we aren't in a position to make sure our poop properly turns into compost - the main reason is that we don't have space to carry a container full of poop around for 1.5 years!

The main issue I have with composting toilets is the misleading marketing. The companies give the impression that when it's time to empty out the poop container, it's dirt that comes out and can be tossed in the yard. They don't straight out and say, "Hey, it will eventually turn into compost, but not after sitting in the poop container for just a week or a month and only if you follow the composting process correctly." I strongly believe that it's the companies' responsibility to provide their potential buyers the full picture of how composting works. I looked through the Nature's Head website and no where does it state that the poop coming out of the toilet needs to be set aside and properly composted for an additional time period. (I'm specifically mentioning them because they seem to be the most popular composting toilet brand among RVers.)

I'm not saying that the designs of composting toilets are defective. I'm pointing out that the poop in the toilet isn't usable composted material by the time it needs to be emptied and the manufacturers don't go out of their way to clarify that.

This is what the contents of our toilet looks like when it's time to empty. (Although, a lot of time we wait too long to empty and it looks a lot worse than this). Hard to see, but there is poop mixed in with the coco pith. This mixture hasn't been composting long enough to kill all the bacteria and turn this into safe, usable soil.

Why The Fuss?

Why am I even talking about this? I felt the need to write this post because I’ve come across RVers who truly believe that their shit ends up as compost by the time they need to empty their composting toilet. This is a problem that concerns all of us if they think what’s in their toilet is safe compost and decide to spread it in the woods (I’ve actually heard and read comments like that). In a lot of cases these people have misinformed, but good intentions - thinking they are giving nutrient rich fertilizer back to the earth. Instead, they are literally tossing their shit in the woods, contaminating the area and water sources with pathogens like E. coli.

Bottom line is: Unless as an RVer, you carry your poop around for 1.5 years in the ideal conditions, you’re not composting anything. So please properly dispose of what empties out of your “composting” toilet, and we will all thank you!